1. Field of the Invention
The present invention relates to an optical multiplexer/demultiplexer of arrayed waveguide grating (AWG) type employable as a wavelength-selecting device in wavelength division multiplexing (WDM) transmission systems.
2. Related Background Art
AWG type optical multiplexer/demultiplexers (hereinafter referred to as AWG circuits) are widely utilized as a wavelength filter allowing a specific wavelength to be taken out or inserted upon interference in WDM transmission systems. Also, AWG circuits as a major optical component in future WDM transmission systems as well as their possibilities of integration with other waveguide devices are expected to develop, since they can be realized by a common fine working process such as lithography or etching without necessitating such precise machining as that of diffraction gratings or such precise multilayer formation as that of interference films.
Such an AWG circuit comprises a structure in which an input waveguide, an input slab waveguide, a plurality of channel waveguides having lengths different from each other (phased array), an output slab waveguide, and an output waveguide are formed integrally on a single substrate.
The inventors have studied the prior art mentioned above and, as a result, have found a problem as follows. Namely, in the case where the AWG circuit is to be made smaller, if it is simply made smaller by improving the precision in its processing, then cross talk may occur between channels to be separated from each other, whereby sufficient spectral characteristics may not be obtained.
FIG. 1A is a view showing a waveguide configuration of an output part in an AWG circuit. In general, this AWG type optical multiplexer/demultiplexer comprises a structure in which a plurality of channel waveguides 10 having lengths different from each other are connected, at their one end, to one connection surface of an output slab waveguide 20 at intervals of d, whereas output waveguides 30 disposed so as to correspond to their respective channel wavelengths of light are connected, at their one end, to the other connection surface of the output slab waveguide 20. FIG. 1B shows a diffraction pattern of light having a channel wavelength corresponding to an output waveguide CH along a line X in FIG. 1A centered at the output waveguide CH.
Usually, when the interval of channel waveguides 10 and the slab length of slab waveguide 20 are designed so as to be secured sufficiently, the diffraction pattern of light to be guided to the output waveguide CH attains a practically unproblematic spectral characteristic in which secondary maximum peaks (side peaks) are relatively small as indicated by curve C1 in FIG. 1B.
In the case where such an AWG circuit is to be made smaller, the relative refractive index difference between the substrate part and waveguide part is set higher, e.g., raised to about 1.5% from its normal value of 0.3% to 0.7%, such that the radiation loss in the channel waveguides 10 becomes smaller even if their radius of curvature is made smaller. At the same time, the slab length of slab waveguide 20 (corresponding to the focal length of the lens surface located at the output end of each channel waveguide 10) and the like are reduced, whereby the AWG circuit as a whole can be made smaller.
If the output end interval d of channel waveguides 10 becomes too small as the AWG circuit as a whole is made smaller, then coupling may occur among the channel waveguides 10. Consequently, at the input end position of an output waveguide CH to take out a light component having a specific phase, a light component having a phase different therefrom arrives, whereby this output waveguide CH would output light in which these components are mixed. Namely, if the output end interval d of channel waveguides 10 is simply reduced, then there may occur a possibility of secondary maximum peaks increasing as shown in curve C2 of FIG. 1B and spectral characteristics remarkably deteriorating.
If the output end interval d of channel waveguides 10 is to be enhanced in order to keep the channel waveguides 10 from coupling with each other, then the number of channel waveguides 10 per se must be reduced. If the number of channel waveguides 10 decreases, then the secondary maximum peaks (side peaks) become greater relative to the main maximum peak (mainpeak), whereby the secondary maximum peaks increase as shown in curve C2 of FIG. 1B as in the above-mentioned case.
In order to overcome the above-mentioned problem, it is an object of the present invention to provide an optical multiplexer/demultiplexer which is made smaller in a state where the cross talk between adjacent channels is effectively restrained from increasing.
For achieving the above-mentioned problem, the present invention provides an optical multiplexer/demultiplexer of AWG type employable as a wavelength-selecting device in a WDM transmission system; the optical multiplexer/demultiplexer comprising a substrate, and one or more input waveguides, a first slab waveguide, a plurality of channel waveguides, a second slab waveguide, and a plurality of output waveguides which are disposed on the substrate.
In the optical multiplexer/demultiplexer according to the present invention, the first and second slab waveguides have respective predetermined slab lengths. Each slab length corresponds to the focal length of the lens surface located at the light input end in the respective slab waveguide. Each input waveguide is a waveguide for guiding to the first slab waveguide a respective signal having a channel wavelength set as a signal channel with a predetermined wavelength spacing, and has one end connected to the first slab waveguide. The plurality of channel waveguides are waveguides, disposed on the substrate, having lengths different from each other. One end of each channel waveguide is connected to the first slab waveguide such that the first slab waveguide is held between the input waveguide and the channel waveguide, whereas the other end of each channel waveguide is connected to the second slab waveguide such that the second slab waveguide is held between the channel waveguide and the output waveguides. Further, the output waveguides are waveguides each having one end connected to the second slab waveguide, and are disposed so as to discretely take out individual signals having channel wavelengths set with a predetermined wavelength spacing.
In the optical multiplexer/demultiplexer according to the present invention, in particular, at least the slab lengths in the first and second slab waveguides and the channel waveguide intervals at respective connection end faces of the first and second slab waveguides are adjusted such that, in a power spectrum indicating a wavelength-transmission characteristic concerning an output light component from selected one of the output waveguides, a second channel wavelength adjacent a first channel wavelength corresponding to the selected one in the channel wavelengths of signals exists within a wavelength region yielding an output light power lower than that at the first channel wavelength by 20 dB or more.
Thus, the slab lengths in the first and second slab waveguides and the channel waveguide intervals at respective connection end faces of the first and second slab waveguides, which greatly contribute to changing the form of the above-mentioned power spectrum, are mainly adjusted, whereby, in each output waveguide, light components other than its corresponding channel wavelength of light, such as light components having wavelengths adjacent the channel wavelength in particular, are effectively attenuated. As a result, cross talk is effectively restrained from occurring between adjacent channels as the optical multiplexer/demultiplexer is made smaller, whereby optical multiplexer/demultiplexers having desirable optical characteristics are obtained.
As modes for adjusting the spectrum form of output light power as mentioned above in relation to each channel wavelength which has been set beforehand as a signal channel with a predetermined wavelength spacing, those in the following can be realized, for example.
In a first mode, the slab lengths and channel waveguide intervals are adjusted such that the second channel wavelength exists between a main peak having a peak wavelength at the first channel wavelength and a first side peak, adjacent the main peak, yielding a power difference of 20 dB or less between the output light power at the peak wavelength thereof and the output light power at the first channel wavelength. Preferably, in the first mode, the slab lengths and channel waveguide intervals are adjusted such that a third channel wavelength, adjacent the second channel wavelength and different from the first channel wavelength, exists between a first side peak and a second side peak, adjacent the first side peak, yielding a difference of 20 dB or less between the output light power at the peak wavelength thereof and the output light power at the first channel wavelength.
In a second mode, the slab lengths and channel waveguide intervals are adjusted such that at least the peak wavelength of a first side peak adjacent a main peak having a peak wavelength at the first channel wavelength exists within a wavelength region between the first and second channel wavelengths. Namely, settings may be such that a plurality of side peaks exist between channel wavelengths adjacent each other.
In each of these modes, the second channel wavelength is set such that the signal wavelength spacing from the first channel wavelength to the second channel wavelength is greater or smaller than the wavelength spacing from the first channel wavelength to the peak wavelength of the first side peak by 20% or more. Namely, if an adjacent channel wavelength (second channel wavelength) is shifted from the peak wavelength of the first side peak by at least a predetermined wavelength spacing, then the cross talk between adjacent channels can fully be reduced in each output waveguide. Preferably, in the first mode, the third channel wavelength is set such that the signal wavelength spacing from the first channel wavelength to the third channel wavelength is greater or smaller than the wavelength spacing from the first channel wavelength to the peak wavelength of the second side peak by 20% or more.
For realizing a smaller size in the optical multiplexer/demultiplexer according to the present invention, it is preferred that the channel waveguide intervals at the respective connection end faces in the first and second slab waveguides be 15 xcexcm or less. Preferably, the slab lengths in the first and second slab waveguides are 15 mm or less.
As the optical multiplexer/demultiplexer is made smaller, the radius of curvature of each waveguide becomes smaller, thereby increasing a possibility that signals propagating through individual waveguides being confined insufficiently. Hence, in the optical multiplexer/demultiplexer according to the present invention, it is preferred that the relative refractive index difference between the substrate and the waveguides though which the signals propagate be 1% or more.
For satisfying the demand for WDM transmissions in recent years, the number of channels corresponding to the number of signals is preferably 30 or more, whereas the channel wavelength spacing of individual signals is preferably 100 GHz or less.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.